Study Of Spin-dependent Transport In Spin Valve And New Spintronic Materials And Structures

Electrons are tiny magnets as well as elementary charged particles. The electron acting as charge makes the last half of the 20^th century be called the microelectronics era. Since the discovery of giant magnetoresistance (GMR) effect in 1988, by incorporating one of these quantum effects, electron spin, into device design, spintronics offers a new dimension to the practice of electronics.The approach to spintronics is very broad and includes the investigation of spin dependent processes in various systems ranging from metallic multilayers via oxide magnets to semiconductors. At present, there are numerous issues which are needed to solve, based on spin-depentent transport in spin valve (SV) and satisfied with the requiring of new material and structure for spintronics, such as achievement large GMR, improving stability and sensitivity, understanding of microscopic origin of exchange bias phenomenon, finding the new materials, structures and methods for combining ferromagnets and semiconductors, etc. In this thesis, design, fabrication, optimized structures of SV and exchange bias have been researched. Furthermore, three kinds of new heterojuctions for spin-dependent transport have been proposed and theoretically analyzed. In addition, based on the research of magnetic half-metallic Fe₃O₄, the SV and Fe₃O₄/n-Si heterojuction has been investigated in detail.The major results are summarized as follows:(1)Following the development of spintronic devices, ferromagnetic semiconductors/normal semiconductors, ferromagnetic semiconductors/ ferromagnetic semiconductors and spin filter I/spin filter II hybrid junctions have been proposed and theoretically analyzed. Theoretical calculation of these junctions showed that spin-dependent transport can be realized in ferromagnetic semiconductors/normal semiconductors junction when a reversal bias voltage is applied. In addition, adtoping ferromagnetic semiconductors/ferromagnetic semiconductors and spin filter I/spin filter II hybrid junctions a large magnetoresistance nearly 100% can be achieved. These new structures give a new way for spintronic material application.(2)The spin valve for spintronic application has been researched for a long time, but there are also a lot of issues which haven't been solved, such as improving GMR effect and sensitivity, expanding switching window, etc. Therefore, based on Mott two spin channel model, the reasons for impacting of GMR effect were studied. Since the magnetic materials, normal metallic material, spin polarization, spin diffusion length, film thickness and resistivity are important for GMR effect, many attempts can be made in fabrication process to improve GMR effect and sensitivity of SV. The basic structure of the SVs Ta/NiFe/Cu/NiFe/FeMn were fabricated. The effect of thickness of free layer, spacer layer, pinned layer, antiferromagnetic layer were investigated, and the optimum growth condition was found. Furthermore the effect of sensitivity and GMR effect by the SVs with different underlayers were studied. As a result, high-sensitivity spin-valves can be achieved by choosing underlayer material with proper surface free energy. Moreover, we have proposed a new type of double exchange-biased SV with two ferromagnetic layers exchange biased in opposite directions by two antiferromagnetic layers. Since the modified SV expanding the switching window of antiparallel state, it causes fabricating the SV more flexible, and especially for the SVs, in which enough large exchange coupling can't be realized at some fabricating conditions.(3)The core structure (ferromagnetic layer/antiferromagnetic layer) of SV multilayer was prepared to investigate the exchange bias variations of the pinned layer, antiferromagnetic layer, microstructure of the films, and the structure of SV. According to the research results, the microscopic origin of exchange bias was analyzed. Furthermore, improvement of exchange bias fields in the same thickness of pinned layer has been found in SV by enhancing the deposition magnetic fields, and 600Oe exchange bias field has been found in SV multilayer with ultra-thin pinned NiFe layers (1-2nm), in which a large constant magnetic field of 650Oe was applied during film deposition procession.(4)In testing of SV, we have first found that a current flowing in a direction not perpendicular to the layer planes of an exchange-biased spin valve systematically changes the exchange bias. The direction of the exchange bias can be completely reversed when a large enough current is applied. This effect occurs only when the direction of the initial sweeping magnetic field is antiparallel to the exchange-bias field. The experimental result not only raises a question over applications such as SV, but also provides a new way to account in detail for the domain structure at an AFM/FM interface.(5)In order to find the large spin polarization new materials for application, half-metallic Fe₃O₄ film has been investigated. The optimum growth condition for stoichiometric and high-crystallographic quality of growth Fe₃O₄ is a sputtering power 200W, annealing temperature 300℃, and deposition under 300Oe external field.(6)Moreover, Fe₃O₄ film was used to fabricate SV. The results showed that GMR effect is very low for Fe₃O₄ spin valve due to the mismatch of the electron densities in ferromagnetic layer and normal metallic layer. In addition, the Fe₃O₄/n-Si heterojuction was also fabricated. The measurements are consistent with theoretical analytic results of ferromagnet/semiconductor junction. This new structure also give a new way for spin injection and transport from magnetic material to normal semiconductor.